• Energy Research

Cubic rock salt can lower down or break the rare earth transition barrier through interstitial or vacancy defects owing to its great deformation and rotation flexibility. Here, we demonstrate that oxygen vacancies in SrO are induced by proper oxidization and atmosphere adjustment, resulting in defects with various depths and crystal field distortion. The thermally assisted tunneling from defects to 5D4 state and electronic population decrease on 5D3 state of Tb3+ are observed by the deformation of adjacent oxygen octahedral structure. Finally, the as-prepared SrO: 0.01 ​Tb3+ phosphors, commercial BaMgAl10O17: Eu2+ blue phosphor, and CaAlSiN3: Eu2+ red phosphor are mixed and coated onto 280 ​nm deep-ultraviolet LED chip to assemble white light-emitting LED device. The LEDs show CCT of 3850 ​K, 4136 ​K, and 4741 ​K, with color rendering index of 90.3, 90.8, and 92.1, respectively. These insights will advance the fundamental knowledge of crystal engineering in cubic rock salt, and enable new ways to manipulate energy transfer and electronic transition via defects.

Redox-active molecules or ions are important in a variety of electrochemical energy storage systems. In lithium-air batteries (LABs), redox-active mediators are added as soluble catalysts that mitigate (dis)­charge overpotentials as well as promote solution-phase reactions that improve the capacity and cycle life of a cell. Redox flow batteries (RFBs) are dependent on the dissolved species to carry and store charge. In both of these systems, crossover phenomena, whereby the redox-active species in solution diffuse from one side of the cell to the other, result in capacity loss. Here, we report a technique to monitor crossover reactions in lithium-air batteries and redox flow batteries, exploiting methodology previously developed to monitor radical formation in redox flow batteries. In this technique, radical concentrations are directly quantified operando by flowing an electrolyte solution containing 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) through nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectrometers. We apply this to Li-TEMPO flow batteries and find that the Coulombic efficiency is only 50%; 50% of the oxidized TEMPO radical, TEMPO + , formed at the cathode crosses over to the anode where it is reduced, regenerating TEMPO. Numerical modeling simulations of static systems cannot capture the extent of redox shuttling seen experimentally unless extremely fast diffusion of TEMPO and TEMPO + is assumed in one-dimensional (1D) models or convection is included in two-dimensional (2D) models, confirming that redox shuttling is enhanced significantly by flow. Finally, we tested Nafion membranes in both flow cells and static LABs and found that the membrane limited crossover of TEMPO and TEMPO + by factors of ∼15× and ∼7×, respectively.